When I first started trading placing stops was extremely difficult for me. Picking the right stop place is hard. Then once you do have a stop in place and the trade drops through it then rebounds to a new 52-week high in the same period - you want to go and hang yourself. I'm convinced that the book manager just sweeps close stops just to grab a quick scalp at the expense of the cowardly trader. That used to piss me off.
Then I realized that you are better off losing the trade to a stop loss than losing money. That's because for every sweep there are 10 instances when the stock drops through and keeps going down.
Now I use a simple rule of thumb - 20 cents for anything up to 10 dollars, 10 cents more per 10 dollars after that. So if stock cost $15 - I use a 30 cent stop, $45 - a 60 cent stop. These are for overnight trades only. For day trades I look at the volatility of the stock and if the stock is very volatile I monitor it and don't put in a stop. If I'm called away from my trading session I put in a tight 20 cent stop regardless of the cost of the trade.
The main rule of stops - don't look back. In other words if you lose the trade to a stop don't check it three or four times during the day because if it is going up you will be depressed and if it continues to go down you might be tempted to reacquire the trade. And if you do you will probably be really depressed later. I speak from experience.